1. Field of the Invention
The present invention is broadly concerned with pressure responsive valve assemblies (which may be of the pressure relief or shutdown type) which are equipped with a rupture disk and a pressure-actuated unit which serves to both actuate the valve and to rupture the disk when the preselected valve set point pressure is exceeded. More particularly, the invention pertains to such valve assemblies, and their use in bypass arrangements, wherein the valve assemblies preferably include small, easily replaceable, concavo-convex, scored forward-acting or reverse buckling metallic rupture disks which can be accurately manufactured to achieve relatively precise, repeatable pressure set points for the valves over a wide range of set points and valve sizes. The valve assemblies may also be constructed so that upon actuation a visual indication of disk rupture can be observed.
2. Description of the Prior Art
Pressure relief or shut down valves have long been provided as a part of safety equipment in a wide variety of situations, e.g., in refineries and chemical processing plants where high pressure fluids are handled. One typical use for relief valves is in bypass assemblies associated with refinery flare lines. Such lines are equipped with butterfly control valves which, if they become jammed or inoperative, can lead to catastrophic failures and explosions. Accordingly, it is common to provide a loop-type bypass conduit around each butterfly valve, with the bypass conduit equipped with an in-line valve or pressure relief device such as a rupture disk. Thus, if a butterfly valve failure occurs, the flare gas is diverted to the bypass line. As pressure builds up in the bypass line it at some point exceeds the burst pressure for the disk or the set point pressure for the valve, whereupon the disk ruptures or the valve operates, so as to open the bypass and relieve the pressure, with the flare gas delivered downstream of the butterfly valve.
The use of in-line burst disks as pressure relief devices in flare bypass arrangements, while safe and effective, requires a relatively complex operation to change out a burst disk. Thus, disk replacement requires unbolting of the bypass piping which can result in spillage of fluids and consequent plant contamination. These difficulties are even more pronounced in the case of large bypass units (e.g., 24-36 inch piping) having correspondingly large disks. Furthermore, in large scale bypass assemblies, the cost of replacement rupture disks becomes significant.
A number of valve designs have also been proposed for use in bypass or similar contexts. One type of valve, exemplified in U.S. Pat. No. 5,297,575 makes use of a pressure-responsive piston which engages an axially extending, collapsible pin. In the event of a pressure rise above the valve set point, the piston is shifted to collapse the retaining pin, thereby allowing valve opening. Similar collapsing pin valve units of the pressure relief or shutdown variety are illustrated in U.S. Pat. Nos. 5,348,039, 5,273,065, 5,012,834, 5,226,442 and 5,373,864.
A significant problem with collapsing pin valve designs is the difficulty in retrofitting the valves to alter the set point pressure thereof. Thus, a given valve unit of this type is provided with pin support structure specific to a particular type of collapsible pin. If it is desired to change the valve set point pressure it is necessary to reconfigure the pin-supporting structure of the valve to accommodate a differently sized pin. This becomes a significant retrofit project, thereby detracting from the usefulness of the collapsing pin valve units.
Valves have also been proposed in the past which make use of pressure-responsive diaphragms which are movable from a normal position to an overcenter position when the valve set point pressure is exceeded. Typical units of this type are disclosed in U.S. Pat. Nos. 4,240,458 and 4,953,579. These valve assemblies are relatively complex and the use of diaphragms makes them difficult to retrofit to alter the valve set point.
European Patent Application 762028 describes a pressure sensitive valve equipped with a piston and rod assembly which serves to reverse buckle a metallic membrane upon valve operation. The inventors assert that it is important that the membrane buckle, as opposed to burst, during operation so as to prevent long term fatigue in the membrane. However, valves of this type are deemed deficient for a number of reasons. First, membrane buckling without bursting thereof severely limits the valve stroke. Hence, with relatively large sized valves and correspondingly larger buckling membranes, the amount of piston and rod travel is limited, thereby limiting the distance that the piston can move off of the valve seat. This limited piston movement may be insufficient to allow rapid and complete venting of the protected conduit or the like. Furthermore, the membranes disclosed in the '028 application are not scored and have angled portions to facilitate buckling (and reversal of the membrane to its original position). This makes the operation of the membrane difficult to calibrate and predict, so that the operation of the valve may be erratic.
There is accordingly a real and unsatisfied need in the art for simplified pressure-responsive valves which can be constructed for precise set point pressure operation while allowing easy field repair and retrofitting thereof without the need for significant disassembly of valve conduits or the valve assembly itself.